In the flotation separation of various minerals, pH plays an important role. Indeed, where possible flotation is carried out in an alkaline medium as most collectors, including xanthates, are stable under alkaline conditions and the corrosion of cells, pipework and the like is minimized. Generally, alkalinity is controlled by the addition of lime, sodium carbonate (soda ash) and, to a lesser extent, sodium hydroxide or ammonia. Acidic compounds such as sulfuric or sulfurous acid are used where a decrease in pH is required.
Generally, lime is used to regulate pulp alkalinity since it is cheap and readily available. It is normally used in the form of milk of lime, which is an aqueous suspension of calcium hydroxide particles. The lime, or alternatively soda ash, is often added to the slurry prior to flotation to precipitate heavy metal ions from solution.
These pH-altering chemicals are often used in significant amounts. Although they are cheaper than collectors and frothers, due to the quantity utilized, the overall cost is generally higher with pH regulators per ton of ore treated than with any other processing chemical. Indeed, the cost of lime for example in many sulfide mineral flotation operations is roughly double that of the collector used.
In conjunction with an appropriate xanthate collector, sufficient alkali will depress almost any sulfide mineral and for any concentration of a particular collector, there is a pH value below which any given mineral will float and above which it will not float. This, of course, allows an operator to selectively float various sulfide minerals from an ore slurry. The "critical pH" value of any ore depends on the nature of the mineral, the particular collector, its concentration and the temperature.
Additionally, lime by itself, or in conjunction with a sulfoxy reagent, acts to depress certain minerals. For example in complex copper/lead/zinc ores, lime with or without sulfoxy reagents acts to depress sphalerite, pyrite and pyrrhotite.
A typical flow chart for a conventional flotation process using lime for pH adjustment is shown in FIG. 1. The slurry is initially mixed with lime in the milling circuit 10. A further pH adjustment 12 may be included where pH is increased to around 9-11, preferably 10.5, by the further addition of lime. A collector and frother may then be added in a reagent conditioning stage 14 followed by the flotation stage(s) 16.
There are, however, difficulties associated with the conventional use of lime and other alkaline agents to alter the pH of the slurry entering a conventional flotation circuit. First, the quantity of lime required in the preparation of the slurry prior to flotation is a significant factor in the cost of preparation. Further, the calcium ions, in the lime, can deposit onto valuable minerals reducing their floatability.
The process provided in accordance with the present invention at least partially overcomes these and other difficulties of the prior art or at least provide a commercial alternative to the prior art.